1. Field of the Invention
This invention relates generally to an input transformer circuit and, more particularly, to a circuit involving transformer-isolated balanced inputs in which the transformer is operated in the current mode.
2. Description of the Background
Professional audio equipment must generally operate to higher standards than conventional consumer equipment. For example, the frequency response must be quite flat in the higher frequency ranges and also the resultant sound must be produced with less distortion than is typically tolerated in equipment for home use. By professional audio equipment this is meant to include stereo mixers as might be employed in a recording studio, as well as preamplification circuits, as might be used with high-quality amplifiers, typically having various signal sources, such as tape inputs, tuner inputs, phono inputs, and microphones, for example. In such multi-input circuits, it is well known to utilize an input transformer to provide the correct impedance match between the respective signal source and the input of the preamplifier mixer.
The inputs to such equipment are usually provided through an input transformer, as described above, and it is known to employ input transformers having a relatively large step-up ratio, that is, the primary to secondary turns ratio is large, on the order of 1:50. The problem inherent in such large ratio step-up transformers is that they suffer from poor high-frequency response and also involve relatively high distortion levels. These problems are principally caused by the high secondary impedance, the leakage inductance, and the winding capacitance of the transformer. To overcome this poor high-frequency response problem, various complicated circuits have been proposed and, indeed, are being employed today to provide high-frequency compensation. Because such compensation circuits are relatively complex, the parts and assembly cost of the overall unit is increased and the possibility of a failure is also increased.